What is the function of amino-acyl-tRNA synthetase?

Aminoacyl tRNA synthetase is an enzyme that participates in charging of tRNA during translation. It helps in binding (esterification) of accurate amino acid to the corresponding  tRNA. 

Process of charging of tRNA or aminoacylation of tRNA is a two step process. 

Step 1: Activation of amino acids in the presence of ATP by Aminoacyl tRNA synthetase

Step 2: Binding of these activated amino acids (in the presence of ATP) to their cognate tRNA.

 Through equations,  it can be represented as 

amino acid + ATP → aminoacyl-AMP + PPi

 aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP

 The charging of tRNA is essential for translation as the start (initiation) codon (AUG) present on mRNA is recognised only by the charged tRNA. When two such charged tRNA are brought closer to each other, the peptide bond formation is energetically favoured and it increases the efficiency of translation.